I did my sons car like this. Removed the lower radiator hoses from radiator. Removed the thermostat housing and kept it attached to the hose. Used a hose nozzle wrapped in a rag and inserted it into the intake where the thermostat sits. Ran water for several minutes. Reversed the flow by shooting the water through the lower hose so it came out the intake. Once it ran crystal clear I disconnected the heater hoses. Used a small nozzle and flushed the heater core and hoses going both directions. I then rinsed out the radiator same method, both directions, until crystal clear. put the lower hose back on the rad and added 1 gallon of 100% coolant. Then added 50/50 mix until it flowed out of intake. Installed new thermostat and closed it all up. Topped it off and bleed it out. Good to go
Don
